1. Field of the Invention
This invention relates to an oil pump rotor assembly used in a trochoid internal gear oil pump which draws and discharges fluid by volume change of cells formed between an inner rotor and an outer rotor when the inner rotor and the outer rotor rotate while engaging each other.
2. Background Art
A conventional oil pump includes an inner rotor having “n” external teeth (hereinafter “n” indicates a natural number), an outer rotor having “n+1” internal teeth which are engageable with the external teeth, and a casing in which a suction port for drawing fluid and a discharge port for discharging fluid are formed, and fluid is drawn and is discharged by rotation of the inner rotor which makes the outer rotor rotate due to engagement of the external teeth and internal teeth, and which produces changes in the volumes of cells formed between the inner rotor and the outer rotor.
Each of the cells is delimited at a front portion and at a rear portion as viewed in the direction of rotation by contact regions between the external teeth of the inner rotor and the internal teeth of the outer rotor, and is also delimited at either side portions by the casing, so that an independent fluid conveying chamber is formed. Each of the cells draws fluid as the volume thereof increases when the cell moves over the suction port after the volume thereof is minimized in the engagement process between the external teeth and the internal teeth, and the cell discharges fluid as the volume thereof decreases when the cell moves over the discharge port after the volume thereof is maximized.
The discharging capacity of such an oil pump could be increased, for example, by increasing the size of the rotors, by increasing an eccentric distance between the rotors so as to increase the volume of each of the cells, or by increasing the revolution rate of the rotors.
However, increase in diameters or thicknesses of the rotors and increase in the revolution rate of the rotors for increasing the discharging capacity are not preferable because increase in diameters or thicknesses of the rotors deviates from the trend in oil pump rotors in which small size is preferred, and increase in the revolution rate of the rotors may cause cavitation which may lead to decrease in pump efficiency, excessive wear, and increase in noise.
On the other hand, when the numbers of teeth of the rotors are reduced, the eccentric distance between the rotors is increased so that the discharging capacity is increased; however, hydraulic pulsation is increased because changes in drawing and discharging flow velocity of oil at the ports are increased and is due to the small number of teeth. As a result, not only does cavitation occur, but also pump efficiency is decreased because oil is drawn from a discharging cell due to excessive negative suction pressure, or because air is drawn through clearance in the casing.
As explained above, the above-described measures are not appropriate to increase the discharging capacity of an oil pump, i.e., such measures cannot fulfill recent requirements of compactness and high performance.